Hydraulic power steering with a closed center

ABSTRACT

A closed-center hydraulic power steering system includes a rechargeable hydraulic pressure accumulator being capable of being connected to a servomotor via controllable pressure-reducing valves which control the motor pressure according to a desired value predetermined by a control device, a connection also being made, if required, between the motor and a relatively pressureless reservoir.

The invention relates to a hydraulic power steering system with

a steering wheel, also

steerable vehicle wheels positively coupled to the latter for steeringadjustment, and

a double-acting hydrostatic servomotor positively coupled to thesteerable vehicle wheels and/or the steering wheel,

a hydraulic pressure-generating device,

a relatively pressureless hydraulic reservoir,

a control-valve arrangement which has two parallel three-waypressure-reducing valves assigned in each case to one side or connectingline of the servomotor and which makes it possible to connect both sidesof the servomotor to the hydraulic reservoir and, in each case, one sideof the servomotor to the pressure-generating device,

a sensor for detecting forces or movements transmitted between thesteering wheel and the steerable vehicle wheels, and also

an electronic control device which is connected, on the input side, tothe sensor and which controls the control-valve arrangement as afunction of the forces or moments transmitted between the steering wheeland steerable vehicle wheels, in such a way that the servomotorgenerates a boosting force in order to reduce a manual force perceptibleon the steering wheel.

BACKGROUND

A power steering system of this type is known from U.S. Pat. No. 5 553683. This known power steering system possesses three valves, theopening state of which is controlled by an electronic control by meansof pulse-width modulation. In this case, the opening and closingbehaviour of the valves is controlled directly, and incorrect settingsof the boosting force may occur at least temporarily.

According to the periodical O +P “Ölhydraulik und Pneumatik” [“OilHydraulics and Pneumatics”] 40 (1996) No. 6, page 409, closed-centerhydraulic power steering systems are known, a high peak power beingavailable if a hydraulic pressure accumulator arranged as a pressuresource is dimensioned appropriately. It is advantageous, furthermore,that the hydraulic steering system consumes energy only when a steeringadjustment of the steerable vehicle wheels is assisted by a boostingforce. By contrast, in the case of a steady-state steering situationwithout any manual force, no hydraulic energy is required.

In a power steering system known from EP-A 0 427 029, the control-valvearrangement is formed by proportionally working control valves which areurged by spring force into a position connecting the respective side ofthe servomotor to the hydraulic reservoir and by a controllableelectromagnet, counter to the spring force, into a position connectingthe respective side of the servomotor to the pressure accumulator.

It is known from U.S. Pat. No. 4,562,710 to provide hydraulic blockingof the servomotor of a power steering system as an immobilizer.

In power steering systems of the type specified in the introduction, asensitive control of the boosting force may present difficulties.

SUMMARY OF THE INVENTION

The object of the invention is, in a steering system of the typespecified in the introduction, to make it possible to improve thecontrol of the boosting force.

This object is achieved, according to the invention, in that

the pressure-generating device has a hydraulic pressure accumulator,

the three-way pressure-reducing valves are provided with a controllablemotor-pressure desired value, and

the control-valve arrangement is controlled by controlling themotor-pressure desired values of the pressure-reducing valves.

The invention is based on the general concept of actuating thecontrol-valve arrangement indirectly by means of the electronic controldevice, in that only the desired value of a motor pressure to be set atthe respective motor connection is predetermined and the valvearrangement otherwise works automatically as a function of thedesired/actual-value deviation of the pressure at the respective motorconnection. The control device thus acts as a desired-value generatorworking as a function of parameters.

In the invention, it is advantageous that the respective motor-pressuredesired values also constitute a measure of the desired value of theboosting force to be set in each case, that is to say thepressure-reducing valves work automatically as a function of thedesired/actual-value deviation of the boosting force.

This, on the one hand, ensures that the boosting force is controlledwithout any delay and as required and, on the other hand, means that no“overreactions” in the control of the boosting forces are to beexpected, even in the event of a rapid change in the operatingconditions.

A particular benefit of the invention is that no hydraulic controlmembers having seals subject to friction have to be arranged in or onthe mechanical drive train or in or on the positive connection betweenthe steering wheel or steering wheel and the vehicle wheels steeredthereby. The invention accordingly ensures a steering system with lowhysteresis in steering manoeuvres with successive steering changes.

Moreover, as regards preferred features of the invention, reference ismade to the claims and to the following explanation of the drawing, bymeans of which particularly preferred embodiments are described.

DESCRIPTION OF THE DRAWINGS

In the drawing:

FIG. 1 shows a general illustration of the steering system according tothe invention in the manner of a circuit diagram,

FIG. 2 shows a functional diagram for the pressure-reducing valves usedin the invention, and

FIG. 3 shows a graph which illustrates, on the one hand, the dependenceof the hydraulic boosting forces as a function of the manual force to beapplied and, on the other hand, the electrical control current at thepressure-reducing valves as a function of the manual force.

DETAILED DESCRIPTION

According to FIG. 1, a motor vehicle, not illustrated in any moredetail, possesses a steering wheel 1 which is positively coupledmechanically to steerable vehicle wheels, not illustrated in any moredetail, for the steering adjustment of these and which, in the exampleillustrated, is drive-coupled, via a pinion 2, to a rack 3, themovements of which are transmitted via track rods, not illustrated, tothe abovementioned steerable vehicle wheels.

The rack 3 forms part of a piston rod of a double-acting hydraulicpiston/cylinder assembly 4, the two sides of which are connected in eachcase separately via connecting lines 5 and 6, in each case to apressure-reducing valve 7 and 8. Each of these is a pressure-reducingvalve with relief, that is to say the pressure-reducing valves 7 and 8can connect the respective side of the piston/cylinder assembly 4, onthe one hand, via intermediate non-return valves 9 and 10, to ahydraulic pressure accumulator 11 or to a relatively pressurelesshydraulic reservoir 12 or can shut off the said side both relative tothe pressure accumulator 11 and relative to the hydraulic reservoir 12.

The pressure-reducing valves 7 and 8 are controllable in terms of theirpressure desired value, as explained in more detail further below.

The pressure accumulator 11 can be charged or constantly maintained at ahigh accumulator pressure of, for example, 120 bar by a pump 13connected on the suction side to the hydraulic reservoir 12, pressurerelief of the pressure accumulator 11 via the pump 13 towards thereservoir 12 being prevented by a non-return valve 14.

A computer-assisted electronic control unit 15 is connected, on theinput side, to a torque sensor 16 which detects the manual force to beapplied on the steering handwheel 1 and, in the illustrated example ofrack-and-pinion steering, is arranged between the steering wheel 1 andthe pinion 2.

Furthermore, the control unit 15 may be connected, on the input side, tovarious additional sensors, for example to a sensor 17 for thetravelling speed of the vehicle.

Moreover, the input side of the control unit 15 is connected to apressure sensor 18 which detects the accumulator pressure of thepressure accumulator 11.

On the output side, the control unit 15 is connected, on the one hand,to control members 7′ and 8′ of the pressure-reducing valves 7 and 8 forcontrolling the respective pressure desired value at the connectinglines 5 and 6 or on the two sides of the piston/cylinder assembly 4.

Moreover, the output side of the control unit 15 controls a drive, notillustrated in any more detail, for example an electric motor for thepump 13.

The arrangement illustrated in FIG. 1 functions as follows:

The piston/cylinder assembly 4 acting as a hydraulic servomotor is togenerate, in each case, a boosting force which is dependent on themanual force on the steering wheel 1, increases with increasing manualforce and decreases with decreasing manual force and assists therespective steering manoeuvre, and the magnitude of which is controlled,where appropriate as a function of parameters, in particular as afunction of the travelling speed detected by the sensor 17. If, forexample, the steering wheel 1 is urged by the driver into a direction ofrotation which corresponds, in FIG. 1, to a displacement of the rack 3to the right, and if, at the same time, the manual force reaches anexcessive value which is “communicated” to the control unit 15 by thetorque sensor 16, then the control unit 15 will set the pressure desiredvalue at the pressure-reducing valve 7 to a relatively high value,whilst the pressure desired value at the pressure-reducing valve 8 isset to a low value or remains at a low value normally set. As a result,the pressure desired value at the pressure-reducing valve 7 willtherefore exceed the pressure actual value at the connecting line 6 oron that side of the piston/cylinder assembly 4 which is on the right inFIG. 1, whilst the pressure desired value at the pressure-reducing valve8 falls below the actual value at the connecting line 6 or on theleft-hand side of the piston/cylinder assembly 4. The pressure-reducingvalve 7 consequently makes a connection between the left-hand side ofthe piston/cylinder assembly 4 and the pressure accumulator 11 which isregularly recharged by the pump 13 being switched on by the control unit15, as soon as the pressure sensor 18 signals a fall below a highpressure threshold value. By contrast, the right-hand side of thepiston/cylinder assembly 4 is connected to the reservoir 12. As aresult, a pressure difference takes effect at the piston/cylinderassembly 4 and seeks to urge the rack 3 to the right in FIG. 1. As soonas the actual/desired-value deviation at the pressure-reducing valves 7and 8 disappears, the respective pressure-reducing valve 7 or 8 assumesa closed position, in which the respectively connected side of thepiston/cylinder assembly 4 or the respective connecting line 5 or 6 isshut off both relative to the pressure accumulator 11 and relative tothe reservoir 12. In the event of any displacement of the rack 3 or ofthe piston of the piston/cylinder assembly 4, the desired/actual-valuedeviations at the pressure-reducing valves 7 and 8 necessarily change,as long as the set pressure desired values remain unchanged;correspondingly, the pressure-reducing valves 7 and 8 are adjusted forthe purpose of reducing the respective desired/actual-value deviation

Since the control unit 15 predetermines the pressure desired values atthe pressure-reducing valves 7 and 8, the boosting force to be generatedby the piston/cylinder assembly 4 is predetermined in terms of amountand direction, the pressure-reducing valves 7 and 8 then automaticallyexecuting the control strokes necessary for achieving or maintaining therespective boosting force.

So that the steerability of the vehicle can be ensured even if there arefaults in the control unit 15, there is expediently provision, whenelectric control current for the control members 7′ and 8′ of thepressure-reducing valves 7 and 8 is switched off, for the saidpressure-reducing valves to assume in each case their operating statefor a minimum pressure desired value, with the result that both sides ofthe piston/cylinder assembly 4 then remain constantly connected to thehydraulic reservoir 12. If, in the event of a fault in the control unit15, the control members 7′ and 8′ are automatically switched tocurrentless, the vehicle can therefore continue to be steered, albeitwithout power assistance.

The system according to the invention may, if appropriate, perform thefunction of an immobilizer. If the control unit 15 does not receive an“authorized driver” signal from a securing unit, not illustrated, beforetravel commences, the pressure desired values at the twopressure-reducing valves 7 and 8 are set simultaneously to a very highmaximum value. The result of this is that the pressure-reducing valves 7and 8 connect both sides of the piston/cylinder assembly 4 constantly tothe pressure accumulator 11, displacement of hydraulic medium betweenthe two sides of the piston/cylinder assembly 4 being prevented by thenon-return valves 9 and 10 and the assembly 4 consequently being blockedhydraulically. The vehicle thus remains unsteerable.

FIG. 2 shows a more functional illustration of the pressure-reducingvalves 7 and 8, only the pressure-reducing valve 7 being reproduced. Infunctional terms, this pressure-reducing valve 7 corresponds to a3/3-way valve which can be changed over between a position in which theconnecting line 5 is connected to the hydraulic reservoir 12, a positionin which the connecting line 5 is shut off both relative to the pressureaccumulator 11 and relative to the reservoir 12, and a position in whichthe connecting line 5 communicates with the pressure accumulator 11. Thevalve body, illustrated in the manner of a slide, is urged by thehydraulic pressure in the connecting line 5 and the force of a firstspring 19 in the direction of the valve position connecting theconnecting line 5 to the reservoir 12. A further spring 20, the springtension of which is variable in a controlled manner by the controlmember 7′, seeks to urge the valve body into its position connecting theconnecting line 5 to the pressure accumulator 11. As a result, therespective valve position is consequently determined by the hydraulicpressure in the connecting line 5, on the one hand, and by the springtension of the spring 20, on the other hand. When the pressure in theline 5 rises sufficiently in relation to the tension of the spring 20,the valve body is set into the position illustrated in FIG. 2. If thehydraulic pressure forces in the line 5 and also the spring tension ofthe spring 19, on the one hand, and the spring tension of the spring 20,on the other hand, are equalized, the valve assumes the middle shut-offposition. If, by contrast, the spring forces of the spring 20predominate, the valve assumes the position connecting the line 5 to thepressure accumulator 11.

Preferably, the control member 7′ is designed to be free ofself-locking, for example as an actuating magnet, so that, when theelectrical control current for the control member 7′ is switched off,relaxation of the spring 20 occurs, with the result that the spring 19sets the valve body into the position illustrated in FIG. 2,irrespective of the hydraulic pressure in the connecting line 5.

FIG. 3 illustrates, on the one hand, the pressures P in each case on oneside of the piston/cylinder assembly 4 as a function of the manual forceM perceptible on the steering wheel 1. In this case, curve K₀ shows theratios with the vehicle stationary (travelling speed =0 km/h), whilstthe curves K₆₀ and K₁₀₀ reproduce the ratios for a travelling speed of60 km/h and 100 km/h, respectively. If a manual force M in one directionis perceptible on the steering wheel 1, the pressure P on one side ofthe piston/cylinder assembly 4 (for example, on the right-hand side) isvaried according to the curves K₀ to K₁₀₀, whilst the pressure on theother side of the piston/cylinder assembly 4 remains insignificantlylow. In the case of a manual force in the opposite direction, thepressure on the other side of the piston/cylinder assembly 4 (forexample, on the left-hand side) is controlled according to theabovementioned curves, whilst the pressure on the first side remainsinsignificantly low.

The curves k₀, k₆₀ and k₁₀₀ show in each case the electrical controlcurrents at the control members 7′ and 8′. In the case of very lowmanual forces M, there is no control current switched on. As soon as themanual force M exceeds a threshold amount in one direction or the other,the electrical control current at in each case one control member 7′ or8′ is increased sharply in order to increase the pressure desired valueat the respective pressure-reducing valve 7 or 8 with increasing amountsof the manual force M. When the amounts of manual force M increase evenfurther, a control of the electrical currents which is similar to thecurves K₀, K₆₀ and K₁₀₀ then takes place.

In contrast to the embodiment illustrated in the drawing, in which thehydrostatic servomotor 4 is designed as a piston/cylinder assembly, arotary hydrostatic motor may also be provided. In the case ofrack-and-pinion steering, a motor of this type may advantageously bearranged on or at the shaft of the pinion 2 and form part of a compactrack-and-pinion mechanism.

What is claimed is:
 1. Hydraulic power steering system with a steeringwheel (1), also steerable vehicle wheels positively coupled to thelatter for steering adjustment, and a double-acting hydrostaticservomotor (4) positively coupled to the steerable vehicle wheels and/orthe steering handle, a hydraulic pressure-generating device (11, 13), arelatively pressureless hydraulic reservoir (12), a control-valvearrangement (7, 8) which has two parallel three-way pressure-reducingvalves (7, 8) assigned in each case to one side or connecting line (5,6) of the servomotor (4) and which makes it possible to connect bothsides of the servomotor (4) to the pressure-generating device (11, 13),a sensor (16) for detecting forces or moments transmitted between thesteering wheel(1) and the steerable vehicle wheels, and also anelectronic control device (15) which is connected, on the input side, tothe sensor (16) and which controls the control-valve arrangement (7, 8)as a function of the forces or moments transmitted between the steeringwheel (1) and the steerable vehicle wheels, in such a way that theservomotor (4) generates a boosting force in order to reduce a manualforce perceptible on the steering wheel (1), characterized in that thepressure-generating device (11, 13) has a hydraulic pressure accumulator(11), the three-way pressure-reducing valves (7, 8) are provided with acontrollable motor-pressure desired value, and the control-valvearrangement (7, 8) is controlled by controlling the motor-pressuredesired values of the pressure-reducing valves (7, 8).
 2. Hydraulicpower steering system according to claim 1, characterized in that thepressure-reducing valves (7, 8) are designed as proportionally workingvalves.
 3. Power steering system according to claim 1, characterized inthat a non-return valve (9, 10), which prevents a return flow to thepressure accumulator (11), is arranged between each pressure-reducingvalve (7, 8) and the pressure accumulator (11).
 4. Power steering systemaccording to claim 1, characterized in that the control device (15)permits to switch on an operating state which can be used as animmobilizer and in which the servomotor (4) is blocked hydraulically. 5.Power steering system according to claim 1, characterized in that thecontrol device (15) predetermines desired values for the motor pressurewhich are varied as a function of parameters.
 6. Power steering systemaccording to claim 1, characterized in that each pressure-reducing valve(7, 8) is urged by a spring force (20) variable by means of the controldevice (15), counter to the pressure at a respective motor connection(5, 6), into a position connecting the pressure accumulator (11) to themotor connection (5, 6).
 7. Power steering system according to claim 1,characterized in that the hydraulic pressure accumulator (11) isassigned an electrical charging pump (13) actuated by the control device(15).